Snails Survive Being Eaten and Excreted by Birds

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Tiny snails can actually benefit from being eaten by birds,
according to a new study that found about 15 percent of the
snails eaten by two species of bird survived their journey
through the birds' guts and out the other end.

The birds appear to spread the snails they have eaten via
excrement, allowing the mollusks to travel much greater distances
than they could
crawling on their own, according to the researchers led by
Shinichiro Wada of Tohoku University in Japan.

The researchers tested out their hypothesis by feeding 174
snails, a species called Tornatellidesboeningi, to four omnivorous birds —three Japanese
white-eyes and one brown-eared bulbul —that also prey upon them
in the wild on the Japanese Ogasawara Islands. [ Album:
Lost Predators Leave Broken Food Chains ]

It took 30 to 40 minutes for the snails, all adults with shells
about 0.1 inches (2.5 millimeters) high, to pass completely
through the birds' digestive systems. An examination of the bird
droppings revealed that 14.3 percent of the snails that had
passed through the white-eyes' guts and 16.4 percent of those
that traveled the brown-eared bulbul's guts were still alive.
Snails that had not moved within 12 hours were considered dead.

The researchers also found genetic evidence indicating that birds
may unintentionally disperse the snails. Gene flow appeared high
among snail populations in different locations; in addition, they
found a positive correlation between the genetic diversity of
snail populations and the density of Japanese white-eyes
on Hahajima Island.

While it is well-known that plants can spread their seeds around
through bird or other animal droppings, it is rare for
animals to employ this strategy. The eggs of zooplankton — tiny
floating animals in the ocean — can pass through fish unharmed,
and birds can also spread wasp larvae. Pond snails that can
survive transit through the fish or ducks that eat them are the
only other known examples of adult animals dispersing this way,
the authors write in the Journal of Biogeography.